1. Field of the Invention
The present invention relates to a method of adjusting the output gradations of an image output apparatus such as an image forming apparatus for forming an image based on image data indicative of the gradations of pixel depths or densities. The present invention also relates to an image output apparatus having an arrangement for automatically adjusting the output gradations. Further, the present invention relates to a method of forming a gradation correction curve to be used in image output apparatuses of the type above-mentioned.
2. Description of the Related Art
Conventionally, there is used a digital color copying machine in which a color document is optically read by a CCD (charge coupled device) scanner or the like and the data thus read are converted into the three primary color signals formed using an additive process of red (R), green (G) and blue (B) and in which, based on the signals thus formed, a color reproduction image of the document is formed. The three primary color signals of R, G and B supplied from the scanner are converted into the three primary color data formed using a subtractive process of cyanogen (C), magenta (M) and yellow (Y) which are respectively the complementary colors of red, green and blue. Each of the three primary color data is an 8-bit data in 256 gradations and indicative of the densities of each color component. Based on the three primary color data of C, M and Y, a black (BK) data is formed.
For example, the surface of a photoreceptor is scanned by a laser beam which is modulated based on the C data, and an electrostatic latent image corresponding to cyanogen is formed on the photoreceptor surface. This electrostatic latent image is developed into a toner image with the use of cyanogen toner, and this toner image is transferred onto a copying paper sheet. Likewise, for the M-, Y- and BK- data, toners of magenta, yellow and black are overlappingly transferred onto this paper sheet, and the toners are finally heated and fixed, thus forming a color copied image.
In view of the nature of a copying machine, an original image can readily be compared with a reproduced image and the demands for the reproducibility are therefore strict. However, a plurality of copying machines individually differ in development characteristics and photoreceptor sensitivity characteristics from one another. Further, the development characteristics vary with the colors of cyanogen, magenta, yellow and black. To faithfully reproduce the colors of an original image, therefore, each copying machine is required to be adjusted for each of the toners in four different colors.
Generally, such adjustment is made by copying a standard color document having an image in standard colors, and by visually comparing the reproduction with the standard color document. Then, there are formed gradation correction curves for correcting the gradations of cyanogen, magenta, yellow and black, respectively, and tables respectively corresponding to these curves are stored in a memory with back-up means in the copying machine.
In such so-called manual adjustment, the adjustment job is troublesome and takes much time. Further, after adjustment, there may possibly be produced variations of output image due to differences among individuals in charge of adjustment.
It is therefore proposed to form gradation correction curves using a data processing unit. However, each of the gradation correction curves is generally a complicated curve which can only be approximated by a curve of the three degree or higher. This disadvantageously causes the calculation to be complicated, requiring a long period of time for forming the gradation correction curves.